1. Field
This invention relates to means for shifting the phase of a signal and, more particularly, to such systems where high accuracy and automatic control are desired.
2. Prior Art
There are three principal classes of prior art system. In the first, the phase shift imparted to a signal of interest is carried out by open loop means. In such systems, there is no measurement of the actual phase shift through the phase shifter, no determination of errors between the command and the actual phase shift imparted, nor any means of correcting such errors.
In the second prior art system, feedback circuitry is incorporated to control the phase shift of a signal of interest, but the phase shift is not measured directly. Instead, a secondary parameter, such as flux, is measured and relied upon to determine the phase imparted by the phase shifter. Unfortunately, there are uncorrected differences between the actual phase and that indicated by secondary parameters. These differences are principally due to changes in the relationship between the phase and the secondary parameter occurring with changes in temperature, time and frequency.
A third class of prior art systems is illustrated in FIG. 1. In this systems, the imparted phase shift produced by the shifter is measured, but it is not a direct measurement of the signal of interest. Instead, the phase of a second or collateral signal is measured. The collateral signal is passed through the phase shifter along with the signal of interest.
In FIG. 1, the signal of interest f.sub.1 and the collateral signal f.sub.c are applied to the input port 101 where they pass through phase shifter 103 to output port 102. The signals applied to the input port are sampled by a directional coupler 104 and supplied to a filter 111. The signals at the output port are similarly sampled by a directional coupler 105 and applied to a second filter 110. The filters 110 and 111 pass only the collateral signals f.sub.c. After passing through filters 110 and 111 the collateral signals are then passed through lines 107 and 109 to phase detector 106. The output of the phase detector is fed back to the control port of phase shifter 103 to adjust the phase imparted to the signal f.sub.1.
In passing through the phase shifter 103, the collateral signal is shifted in phase by an amount .phi. which is similar to the shift in phase of the signal of interest f.sub.1. For the phase shift to be similar, the parameters of the phase shifter 103 must be uniform across the frequency range between f.sub.1 and f.sub.c. If the path length, represented symbolically as L.sub.1 (107), from the input port through the filter 111 to the phase detector, were equal to the path length L.sub.2 (109), from the input port through the filter 110 to the phase detector, the difference in phase between the two samples of the collateral signal would be due to the phase shifter and would be measured directly by the phase detector. However, the path lengths are intentionally offset by a definite amount referred to as the reference length 108. As the frequency of f.sub.c is changed, the phase reaching the phase detector is changed because of the offset in line lengths. By changing the frequency of the collateral signal, the control signal produced at the output of the phase detector will change, causing the phase shifter to impart a phase shift to the signal f.sub.1 determined by the frequency f.sub.c.
The operation of the feedback circuit shown in FIG. 1 is dependent on the type of phase shifter used. There are two types of electronically controlled phase shifters in common use. The first is the latching phase shifter which is typically a ferrite phase shifter. In this type of shifter the phase is continuously changed for as long as the control signal is applied. The phase stops shifting when the signal is discontinued.
The second type of phase shifter is the nonlatching phase shifter which is typically a varactor phase shifter in which the phase shift is dependent upon the magnitude of the applied control voltage. If the voltage is removed, the phase will change to that which corresponds to zero voltage.
For the system of FIG. 1 to operate with a latching type phase shifter, the phase detector output must go to zero once the phase shifter 103 imparts the desired phase angle. For the sake of phase comparison, the effective phase shift for the portion of the collateral signal passing through filter 111 is due to the reference length 108, while the effective phase shift for collateral signal passing through filter 110 is .phi. imparted by the phase shifter, as the remaining lengths in each path are taken as equal. When the phase shift due to the reference line 108 is equal to the phase shift through the phase shifter 103, the phase detector will receive two signals at the same phase angle and produce a zero voltage output signal. This will stop any further change in the shifter 103. The phase shifter will then have been set to the angle determined by the frequency of the collateral signal and the path length of the reference line. Drift from this phase angle by the shifter will produce a difference in phase at the inputs to the phase detector and a corresponding corrective control voltage at the output of the phase detector.
Although not shown as separate components, all systems illustrated in the Figures are considered as having the usual amplification, buffering and shaping networks contained within the phase detector to drive the phase shifter to the desired position as is normally required in a feedback control network.
Where a nonlatching phase shifter is used, the feedback operation is similar except a slight offset from zero output from the phase detector will be amplified by amplifiers within the phase detector to set the phase shifter 103 to the desired phase angle. The stable point for this feedback systems occurs where the phase shifter 103 is essentially at the desired angle.
A variation of the system shown in FIG. 1 is shown in FIG. 2. In this Figure, the signal of interest f.sub.1 is passed from input port 201 through phase shifter 203 to output port 202. A sample of the input signal f.sub.1 is coupled through directional coupler 204 to a first mixer 211, while a portion of the output signal is coupled through a second directional coupler 205 to a second mixer 212. The local oscillator signal for both the first and second mixers is a collateral signal f.sub.c. The output of the first and second mixers is supplied to the phase detector 206 which produces at its output a control signal for the phase shifter 203.
In a manner similar to that shown in FIG. 1, the basic line lengths to the phase detector are equal. That is, L.sub.1 (207) is equal to L.sub.2 (209) and there is a reference length 208 added in series with the output of the first mixer 211 to provide a phase shift with frequency. As the collateral signal f.sub.c is varied, the frequency of the output from the mixers varies and the unequal line lengths due to the addition of reference line 208 produces a shift in phase used to control the phase of the shifter in the same manner as line length 108 in FIG. 1.
The circuit of FIG. 2 has several advantages over the circuit of FIG. 1. Only a single signal, f.sub.1 is applied to the input port and passed through phase shifter, eliminating the need for the phase shifter to have a uniform phase across a frequency range. Also eliminated is the need for filters and for injecting two signals into the input ports simultaneously.
On the other hand, the circuit of FIG. 2 has the disadvantages of requiring two mixers, a collateral signal source and a means of supplying it to the two mixers. In the circuits of FIG. 1 and of FIG. 2, the phase of the signal of interest f.sub.1 is not measured directly. As a result, the circuitry suffers in accuracy, complication and expense.